Field of the Invention
Embodiments of the invention generally relate to apparatuses and methods for the fabrication of solar, semiconductor, and electronic materials and devices, and more particularly to epitaxial lift off (ELO) thin films and devices.
Description of the Related Art
One phase in device fabrication involves handling and packaging of thin films used as photovoltaic or solar devices, semiconductor devices, or other electronic devices. Such thin film devices may be manufactured by utilizing a variety of processes for depositing materials onto and removing material from a substrate or wafer. One uncommon technique for manufacturing thin film devices is known as the epitaxial lift off (ELO) process. The ELO process includes depositing an epitaxial layer or film on a sacrificial layer on a growth substrate, then etching the sacrificial layer to separate the epitaxial layer from the growth substrate. The removed thin epitaxial layer is known as the ELO or epitaxial film or layer and typically includes thin films used as photovoltaic or solar devices, semiconductor devices, or other electronic devices.
The thin ELO films are very difficult to manage or handle, such as when bonding to a substrate or while packaging, since the ELO films are very fragile and have narrow dimensions. The ELO films crack under very small forces. Also, the ELO films are very difficult to align due to their extremely narrow dimensions.
The sacrificial layer is typically very thin and may be etched away via a wet chemical process. The speed of the overall process may be limited by the lack of delivery or exposure of reactant to the etch front, which leads to less removal of by-products from the etch front. The ELO etching process is a diffusion limited process and when the ELO films are maintained in their as deposited geometries, a very narrow and long opening forms which severely limits the overall speed of the process. To lessen the transport constraint of the diffusion processes, it may be beneficial to open up the resulting opening created by the etched or removed sacrificial layer and bending the ELO film away from the growth substrate. The act of bending while etching forms a crevice between the ELO film and the growth substrate—which geometry of the crevice provides greater angles to increase the transport of species both towards and away the etch front. Reactants move towards the etch front while by-products generally move away from the etch front.
The bending of the ELO film however can induce stresses the epitaxial layers within and the amount of bending is limited by the strength of the ELO film. The ELO film usually contains a brittle material, which does not undergo plastic deformation before failure, and as such may be subject to crack induced failures.
To minimize the potential for crack propagation, the brittle ELO film may be maintained under a compressive stress. Cracks usually do not propagate through regions of residual compressive stress. The ELO film is placed under tensile stress while bending the ELO film away from the growth substrate since the ELO film is on the outside of the curvature of the crevice. The tensile stress limits the amount of crevice curvature and reduces the speed of the etch process. To overcome this limitation, a residual compressive stress may be instilled within the ELO film before etching the sacrificial layer. This initial compressive stress may be offset by tensile stress caused by the bending and therefore allows for a greater amount of bending during the separation process.
Also, the ELO process has always been a cost prohibiting technique for commercially producing the thin ELO film devices. Current ELO processes include transferring a single growth substrate through many fabrication steps while producing a single ELO film. The current processes are time consuming, costly, and rarely produce commercial quality ELO films.
Therefore, there is a need for more effective, less time consuming, and less expensive methods and apparatuses to remove and handle ELO thin films.